With a demand for weight reduction and power enhancement in automobiles or the like, valve springs for use in engine, suspension springs or the like have been designed for high-stress use. Further, in relation with increase in load stresses acting on springs, there is also the need for springs having enhanced fatigue strength and sag resistance.
Recently, it has been a custom that most valve springs or suspension springs are produced by subjecting a quenched and tempered steel wire, so-called “oil-tempered wire”, to a coiling process at room temperature.
Since the aforementioned oil-tempered wire has a tempered martensite structure, it has advantages of conveniently providing high strength and exhibiting excellent fatigue strength and sag resistance. However, it involves a problematic requirement of a large-scale facility and a high process cost associated with heat treatments, such as quenching and tempering treatments.
Some of springs designed to have a relatively low upper limit on load stresses is produced by drawing carbon steel with a ferrite-pearlite structure or a pearlite structure to obtain a wire having enhanced strength (so called “hard-drawn wire”), and by coiling the hard-drawn wire at room temperature. According to the JIS, such a spring is defined as “Piano Wire SWP-V”, particularly “for valve springs or springs similar thereto”, in JIS G 3522 “Piano Wires”.
Springs made of the hard-drawn wire as above (hereinafter, referred to as “hard-drawn spring”) are advantageously obtained at a low cost because of no need for heat treatments. However, since those conventional hard-drawn wires provide only hard-drawn springs with low fatigue strength and sag resistance, they cannot provide for high-stress springs which are increasingly required in recent years.
There also have been studied various techniques for providing hard-drawn springs for high-stress use in light of the advantage of low-cost production. For example, Japanese Unexamined Patent Publication No. 11-199981 proposes an exemplified method for obtaining cementite of a specific configuration by performing a wire drawing process to pearlite in eutectoid-hypereutectoid steel, which is usable as “a piano wire having properties equivalent to an oil-tempered wire”. However, this method inevitably involves increase in production cost due to complicated process, such as changing of drawing direction which is additionally required.
In view of the above, an object of the present invention is to provide a steel wire used for producing hard-drawn springs capable of exhibiting fatigue strength and sag resistance equivalent or superior to springs produced using an oil-tempered wire, and a hard-drawn spring produced using such a steel wire.